Vibration of the apical oxygens in mercury-based superconductors 1

Mercury-based superconductors HgBa₂Ca_n-1Cu_nO_2n+2+δ, Hg-12(n-1)n, show strong Raman peaks at ∼ 570 cm^-1 and ∼ 585 cm^-1. The mode at ∼ 585 cm^-1 is unambiguously the A_1g mode of the apical oxygen O_A. However, the origin of the mode at ∼ 570 cm^-1 is still in question. In order to clarify the origin of the 570 cm^-1 mode, we have done Raman measurements on the Tl-doped Hg-1223 samples, where the amount of the oxygen O_δ in the Hg/Tl-O plane is controlled by the doping level of Tl ions. The results show that the 570 cm^-1 mode is not from the motion of Oδ, but from the A_1g-type motion of O_A- This is in contradiction with the factor group analysis of the ideal Hg-12(n-1)n structure with fully-empty or fully-occupied O_δ position, which shows that there is only one A_1g mode for O_A- The result indicates that, in the Hg12(n-1)n systems with partially-occupied Oδ, there are two possible environments for O_A; one with O_δ(neighbors, the other without any nearest O_δ neighbor.